Previously, settling tanks for the separation of solid particles from liquid which tanks also included filters, often had filters with vertically extending parallel tubes or plates, which filters became covered and/or bridged with the cakes of particles that did not detach from the socks on the filters, or fall off and settle to the bottom of the settling tank for removal. Furthermore these filters either had to be separately removed or they were mounted in a custer in a base which then could be removed for cleaning and replacement, but no protection for the filters themselves was provided, and very careful handling of the filters was required to avoid puncturing of the socks which were usually used over them. Also the whole system had to be shut down, the tank drained and even the piping disconnected in order to replace these filters.
Also if a precoat medium was used on such filters, it required a relatively long time for circulation of the liquid in the tank until substantially all of the precoat medium suspended in the liquid was deposited on the filters and much of it was lost by settling in the bottom of the tank.